Note: Descriptions are shown in the official language in which they were submitted.
"` ~22S501
Back~round of the Invention
The present invention broadly relates to the
autocatalytic chemical deposition o~ nickel, and more
particularly, to an improved aqueous electroless nicXel
plating bath and process for depositing nickel on a sub-
strate.
A variety of nickel containing aqueous solu-
tions have heretofore been used or proposed for use for
chemically depositing nickel on a substrate incorporating
various additive components for controlling the rate of
nickel deposition and for promoting stability of the bath
after prolonged usage. Among such compositions are those
such as disclosed in United States Patents No. 2,762,723;
2,822,293, and 3,489,576. In addition to a controlled
concentration of nickel ions, such prior art electroless
nickel plating baths conventionally employ hypophosphite
anions for reducing the nickel cation to the metallic
state and the hypophosphite anions are in turn oxidized
to phosphite anions and other deg~adation products some of
which combine with other nickel ions present in the solu-
tion forming a finely particulated dispersion producing a
random chemical reduction of the other nickel ions present
- in the bath cau~ing the resultant nickel deposit on the
substrate to become progressively coarse, rough and some-
times porous. The presence of such
.I`
~ ~225501
fine-sized dispersed particulate matter also promotes instability
of the chemical balance of the bath ultimately resulting in a
deoompositian thereof ne oe ssitating discarding the bath and
repla ~.e~.t.
For these and other reasons, various additive agents as
described in the aforementioned United States patents have
heretofore been employed or proposed for use to stabilize the bath
and to further oontrol the rate of nickel deposition on a
substrate being plated. In such electroless nickel plating baths
emplc~ing hypcphosphite ions as the reducing agent, the nickel
deposit actually ccmprises an alloy of nickel and phosphorus with
the phosphorus content usually ranging from about 2 to about 15
percent by weight. The physical and chemical prcperties of such
nickel-phosphorus alloy deposits are related to the percentage of
the phosphorus present and in turn, the percentage of phosphorus
in the deposit is influenced by a number of factors including the
bath operating temperature, the operating pH, the hypophosphite
ion conoentration, the nickel ion concentration, the phosphite ion
and hypophosphite degradation product concentration as well as the
total chemical composition of the bath including additive agents.
In end uses of electroless nickel plated articles, those
applications requiring maximum deposit hardness or nickel deposits
which are nonmagnetic, it is normally necessary to provide nickel
alloy deposits with a relatively high peroentage of phosphorus
such as 9 percent by weight or greater. However, there are
numerous other applications for electroless nickel-phosphorus
alloys in which a lower percentage of phosphorus is desirable and
i2:~55(1 1
an Aerospace Material Specification, P~IS2405A provides for
nickel-phosphorus alloy deposits in which the phosphorus content
is to be held to a minimum and, in any event, shall not exceed 8
percent by weight.
Prior art compositions and processes for producing
nickel-phosphorus alloy deposits having low percentages of
phosphorus have been found susceptible to producing bath
instability, a shortening of the operating life of the kath and/or
have caused increased difficulty to control the bath because of
the relatively narrcw concentration ranges of some of the bath
constituents. For example, the addition of thiourea to an
electroless nickel bath has been found effective to reduce the
phosphorus content in the resultant nickel deposit. Hcwever, at a
concentration of between 2.5 and 3 parts per million t33 to 40
micro mol per liter), thiourea causes such bath formulations to
cease plating. It has been reported that the critical narrow
concentration limits of thiourea in an electroless nickel plating
bath to provide satisfactory operation renders this additive agent
impractical for commercial plating installations because analysis
and replenishment of such baths to maintain proper co~position
parameters is difficult, time consuming and cost intensive.
Alternative sulfur-containing organic additive agents
have been praposed for stabilizing and/or increasing the
deposition rate of nickel from electroless nickel plating baths
such as described in United States Patents Nos. 2,762,723 and
3,489,576. Such alternative additive materials have also been
found oommercially impractical because of a very narrow useful
122550~
concentration range and moreover, many of such sulfur-containing
organic compounds do not produce a nickel-phosphorus alloy deposit
in which the phosphorus content is belcw about 8 percent by
weight.
Prior art compositions and processes for pro*ucing
nickel-phosphorus alloy deposits of relatively high phosphorus
contents have also been subject to the disadvantages of requiring
relatively rigid control of the concentration of the bath
constituents detracting from the ease of control, maintenance and
replenishment of such baths to maintain optimum operating
performance. The use of stabilizing agents for prcviding
increased bath stability has occasioned in prior art compositions
a condition of over stabilization whereby a cessation of plating
occurs. In such instan oe s, it has been necessary to discard the
bath and prepare a new operating bath which constitutes a costly
and tinE-consuming operation.
me present invention provides for an improved
electroless nickel plating bath and process for depositing a
nickel-phosphorus alloy of relatively low phosphorus oontent
incorporating an additive agent which can satisfactorily be
employed over a relatively broad cperating concentration range
while at the same time increasing the rate of deposition of the
nickel by as much as 30 percent or mDre. The present invention
~urther provides for an improved electroless nickel plating bath
and process suitable for use in depositing nickel-phosphorus
platings of relatively high phosphorus content providing for
greater latitude in variations in the bath constituents thereby
12Z5S01
achieving simpler oontrol and facilitating maintenan oe and
repl d shment of the bath. The present invention further
contempLates a method for rej wenating or resboring an electroless
nickel plating bath which has been rendered inoperative due to
o~er stabilization thereof by inclusion of organic and/or
inorganic stabilizi~g agents in excessive amounts by the addition
of an additive agent of the present invention wherehy satisfactory
operation of the bath is restored.
Summary of the Invention
The benefits and advantages of the present invention are
achieved in acoordan oe with the composition aspects thereof by an
acueous electroless nickel plating bath oontaining nickel ions,
hypcphosphite ions, and an, am~unt of a sulfonium betaine
ccmpaund sufficient to control the rate of nickel deposition and
the con oe ntration of phosphorus in the nickel deposit. m e
sulfonium betaine compound corresponds to the structural formula:
,,, Rl~
C-S-(CHR)--S03e
R4 -
Wherein:
P~ and R4 are the sa~e or different and are
H, Cl-C6 alkyl radicals, Cl-C6 hydroxy alkyl
radicals,
R is the same or different and is H or aH, and
- n is an integer of f.~l 1 to S,
as well as mixtures thereof.
~2~55~1
m e oonoentration of nickel ions generally ranges from abcut 1 to
about 15 grams per liter (g/l), the h~xqphosphite ions range from
about 2 to a~out 40 g/l and the sulfonium betaLne cc~pound can
range from about 1 up to about 200 micro mol per liter. The bath
in order to prc~ide satisfactory prolonged commercial operation
further incorporates a cc~plexing agent usually present in an
amount up to about 200 g/l for ccmplexing the nickel ions present
as well as to solubilize the hypophosphite degradation products
formed during prolonged usage of the bath. The electroless bath
desirably further contains a buffering agent generally present in
amounts up to about 30 g/l, a wetting agent to minimize surfa oe
pitting, usually present in an amount up to a~out 1 g/l and
hydrogen or hydroxyl ions to provide a bath on the acid or
aIkaline side as may be desired. Optionally, but preferably, the
bath further ~ ~loys in combination with the sulfonium betaine
compound at least one supplemental organic or inorganic stabilizer
agent of the various types heretofore known which can be employed
in amounts up to that level at which the rate of deposition of
nickel is undesirably impaired.
In accordance with the process aspects of the present
invention, a low phosphorus-nickel alloy is deposited on a
metallic or non~metallic substrate by contacting the cleaned and
suitably prepared substrate with the electroless nickel bath at a
temperature generally ranging from about 40C up to boiling for a
period of as little as 1 minute up to several hours or even days
to prcvide a nickel-phosphorus alloy deposit of the desired
thickness. During the deposition process, agitation of the bath
., ~2SS~l
is preferred, emplcying mild air or other forms of mechanical
agitation. The bath is also preferably subjec~ed to periodic or
continuous filtration to re~ove solid contamunants. The bath is
periodically and/or oontinuously replenished for maintaining the
bath constituents within the desirable operating concentrations
and at the appropriate pH level.
The present invention further contemplates a process for
effecting rejuvenati~n of an electroless nickel bath which has
been rendered inoperative due to an over stabilization thereof by
organic and/or inorganic stabilizing agents by the addition of a
controlled effective amDunt of a sulfonium betaine compound to
restore operation the.reof.
Additional benefits and advantages of the present
invention will be~a.~ apparent upon a reading of the Description
of the Preferred Embodiments taken in conjunction with the
accompanying examples.
.
Description of the Preferred Embodiments
The aqueous electroless nickel plating baths of the
present invention can be cperated over a broad pH range including
the acid side and the aLkaline side at a pH of from about 4 up to
about 10. For an a~idic bath, the pH can generally range from
about 4 up to abcut 7 with a pH of about 4.3 to about 5.2 being
preferred. For an aLkaline bath, the pH can range from about 7 up
to about 10 with a pH range of from about 8 to about 9 being
preferred. Since the bath has a tendency to become more acidic
during its operation due to the formation of hydrcgen ions, the pH
~ 12Z55(~1
is periodically or continuously adjusted by addinq bath soluble
and oo~patible aIkal~le substances such as alkali metal and
ammonium hydroxides, carbonates and bicarbonates. Stability of
the cperating pH is also provided by the addition of various
buffer compounds such as acetic acid, propionic acid, boric acid
or the like in amounts up to about 30 g/l with amounts of about 4
to about 12 g/l being typical.
The nickel ions are introduced into the bath employing
various bath soluble and compatihle nickel salts such as nickel
sulfate hexahydrate, nickel chloride, nickel acetate, and the like
to provide an operating nickel ion concentration ranging from
about 1 up to about 15 g/l with concentrations of from about 3 to
about 9 g/l being preferred and with a concentration of about 5 to
about 8 g/l being opti~num. The hypophosphite reducing ions are
introduced by hypophosphorous acid, sodium or potassium
hypophosphite, as well as other bath soluble and compatible salts
thereof to provide a hypophosphite ion concentration of about 2 up
to about 40 g/l, preferably c~bout 12 to 25 g/l with a
con oe ntration of cibout 15 to about 20 g/l being optimum. The
specific concentration of the nickel ions and hypophosphite ions
employed will vary within the aforementioned ranges depending upon
the relative concentration of these two constituents in the bath,
the particular operating conditions of the bath and the types and
concentrations of other bath components present.
In order to provide a commercially satisfactory plating
bath of reasonable longevity and operating performance, it is
conventional preferred practice to incorporate a complexing agent
.` 1~2SSOl
or mixture of oomplexing agents in amDunts sufficient to ~ul~lex
the nickel ions present in the bath and to further solubilize the
hypophcsphite degradation products formed during usage ~of the
bath. The oomplexing of the nickel ions present in the bath
retards the formation of nickel orthc~hosphite which is of
relatively low solubility and tends to form insoluble suspensoids
which not only act as catalytic nuclei promoting bath
decomposition but also result in the formation of coarse or rough
undesirable nickel deposits. Generally, the cc~plexing agents are
employed in amounts up to about 200 g/l with amounts of about 15
to about 75 g/l being preferred while amDunts of about 20 to abcut
40 g/l are typical. Complexing or chelating agents of the various
types described in the aforementioned United States patents, the
teachings of which are incorporated herein by reference, can be
satisfactorily employed for this purpose and the particular
selection of such c~nplexing agent or nNLYture of complexing agents
will be dependent to some extent on the operating bath pH to
prc~ide complexors of maximum stability under such specific pH
conditions. Typical of such cc~plexing agents are the acid as
well as aIkali metal and anmDnium salts of glycolic acid, lactic
acid, malic acid, glycine, citric acid, acetic acid, tartaric
acid, succinic acid, and the like. While aIkaline earth metal
salts can also be employed to some extent, the tendency of such
alkaline earth metals to fonm insoluble precipitates with the bath
constituents renders them less desirable and for this reason are
preferably excluded. It will also be appreciated, that certain
complexing agents such as acetic acid, for example, also act as a
"-` 12Z~5~31
buffering agent and the appropriate concentration of such additive
oomponents can be cptimized for any bath oomposition in
consideration of their dual-functioning properties.
In a~ition to the foregoing constituents, the bath
further includes as an essential constituent, a plating rate and
phosphorus controlling agent present in an amcunt effective to
enhan oe the rate of deposition of the nickel-phosphorus allcy and
to provide an alloy deposit generally containing less than ab~ut 8
per oe nt by weight phosphorus. Ihe additive agent comprises a
sul.onium betaine oompound corresponding to tke structural
formLla:
Rl~
~3~C-S- (CHR)--S03e
R?N
~herein:
P~, R2, ~ and R4 are the same or different and are
H, Cl-C6 alkyl radicals, Cl-C6 hydroxy al~yl
radicals,
R is the same or different and is H or OH, and
n is an integer of fro~ 1 to 5,
as well as mixtures thereof.
A sulfonium betaine ocmpound oorresponding to the
foregoing structural formula which has ~een found particularly
satisfactory comprises 3-S-isokhiuronium propane sulfonate. Ihis
~,yound corresponds to tke foregoing structural formula in which
`-` lZZ5501
Rl, R2~ R3 and R4 are hydrogen and n is 3. Alternative
satisfactory ~sulfonium betaine compounds which can be emplc~d
include N,N'-dimethyl-3-S-isothiuronium propane sulfonate,
N,N'-diethyl-3-S-isothiuronium propane sulfonate,
N,N'-di~ydroxymethyl-3-S-isothiuronium propane sulfonate,
N,N'-diisopropy1-3-S-isothiuronium propane sulfonate,
N,N,N',N'-tetramethyl-3-S-isothiuronium propane sulfonate,
N,N,N'-trimethyl-3-S-isothiuronium propane sulfonate,
2-S-isothiuronium ethane sulfonate, 3-S-isothiuronium propane-2-ol
sulfonate and the like. These additive compounds are extremely
effective even in relatively l~ow concentrations such as about 1
micro mol per liter to oon oe ntrations as high as about 200 micro
mol per liter. The foregoing broad range of operating
concentrations provides for a substantial simplification of
analysis and control of the operating bath under c~~ rcial
operating conditions providing significant advantages over prior
art additiv~ ccmpounds of the types heretofore kncwn. The
sulfonium betaine oompound in accordance with a preferred practice
is employed in acidic baths at a concertration of about 10 to
about 150 micro mol per liter with a concentration of about 20 to
about 120 micro mDl per liter being typical. In aLkaline baths,
the sulfonium betaine compound is preferably employed at a
con oe ntration of about 1 to about 50 micro mol per liter with a
concentration of about 2 to about 25 micro mol per liter being
typical.
In accordance with a further preferred practice of the
present invention, the sulfonium betaine compound is e~ployed in
.
;50~
combination with other conventional inorganic and/or organic
stabilizing agents of the types heretofore kncwn including lead
ions, cadmlum ions, tin ions, bi~muth ions, antim~ny ions and zinc
ions which can conveniently be introduced in the fonm of bath
soluble and compatible salts including halides, acetates,
sulfates, and the like. Plternatively, otner stabilizing agents
can be employed including cyanide ions, thiocyanate ions, and the
like which typically can be used in amounts of from about 1 up to
about 20 p~m. Lead ions can be employed in amounts usually up to
about 2 ppm; cadmium ions in an amount up to about 10 ppm;
antimony and tin ions can be employed in an amount up to about 100
ppm. m e specific concentration of such supplemental stabilizing
agent or mixtures of supplemental stabilizing agents is limited by
such con oe ntrations at which the rate of nickel deposition is
inhibited to an undesirable magnitude rendering the bath
commercially impractical.
The bath may additionally employ one or a mixture of
suitable wetting agents of any of the various types heretofore
known which are soluble and compatible with the other bath
aonstituents. The use of such wetting agents is desirable to
prevent pitting of the nickel alloy deposit and can usually be
employed in amounts up to about 1 g/l.
In acaordance with the pro oe ss aspects of the present
invention,-a substrate to be plated is contacted with the bath
solutian at a temperature of at least about 40C up to boiling.
Electroless nickel baths of an acidic type are preferably employed
at a temperature of from about 70 to about 95C with a temperature
" ~ZZ55C~:l
of about 80 to a~out 90C bein~ optimum. Electroless nickel
baths on the alkaline side are generally operated within the broad
operating range but at a correspondingly lower temperature than
the acid-type bath sin oe pH and bath temperature are inte..~lated
in that the rate of nic~el deposition increases as the pH
increases but the stability of the bath increases as the pH of the
bath decreases while the rate of deposition of nickel increases as
the temperature increases but with a corresponding decrease in
bath stability.
me duration of contact of the electroless nickel
solution with a substrate being plated is a function dependent
entirely on the desired thickness of the nickel-phosphorus alloy
desired. Typically, the contact time can range frcm as little as
about 1 minute to several hours or even several days.
Conventionally, a plating deposit of about 0.2 up to about 1.5
mils is a normal thickness for many commercial applications. When
wear resistance is desired, such deposits can be applied at a
thickness of about 3 to about 5 mils such as on valves, pipes,
dies and the like. m icknesses of up to about 0.25 inch can also
be achieved by a correspondingly longer contact time as may be
desired.
During the deposition of the nickel alloy plate, it is
preferred to employ mild agitation such as mild air agitation,
mechanical agitation, bath circulation by pumping, as well as by
bar~el plating in which the rotation of the immersed barrel
imparts agitation to the batn. It is also preferred to subject
the bath to a periodic or continuous filtration treatment to
~ZZS5~1
. ,~
reduce the level of contam~nants therein. Replenishment of the
constituents in the bath is also performed on a periodic or
continuous basis to maintain the con oe ntration, particularly of
the nickel icns, hypaphosphite ions, and p~ level within the
desired limits.
The substrate to be plated is subjected to a prellminary
surfa oe preparation in accordan oe with oonventional practioe to
provide a clean and catalytically active surface. In the case of
substrates which Q nnot be directly coated employ mg the
electroless nickel bath because of their non-catalytic nature
relative to the chemistry of the bath, the substrates can be
preliminarily subjected to an electrolytic plating of nickel or
such other metal which is catalytic whereby the substrate surfa oe
is re oe ptive or made receptive to chemical deposition of nickel
fron the electroless bath.
In acc~rdan oe with a further process aspect of the
present invention, electroless nickel plating baths which have
been rendered inoperative for depositing a nickel-phosphorus alloy
deposit on a substrate due to the use of an excessive amount of
inorganic and/or organic stabilizing agents, can be restored to
effective operation by the addition thereto of a sulfonium betaine
ccr~x~m d as hereinbefore defined as ~ell as mixtures thereof in an
amount effective to restore satisfactory operation. The
concentration of the sulfonium betaine compound employed for
effec,ting rejuvenation can range within the limits as hereinbefore
specified with ooncentrations of from about 20 to about 120 micro
mol per liter being typical for acidic-type baths and with
.~
14
-
, ~Z25SOl
con oe ntrations of about 2 to about 25 micro mol pPr liter being
typical for alkaline-type baths. m e sulfonium betaine oompound
is added bo the bath in the presen oe of agitation to effect a
substantially unifonm distribution thereof.
In order to further illustrate the present invention,
the following examples are p m vided. It will be understood that
the examples are provided for illustrative purposes and are not
intended bo be lLmdting of the soope of the present Lnvention as
herein described and as set forth in the subjoined claims.
E~MPLE 1
Three 500 milliliter electroless nickel plating baths
were prepared oontaining 27 g/l of nickel sulfate hexahydrate
(equivalent to 6 g/l nickel ions); 24 g/l of sodium hypophosphite
mDnohydrate (equivalent to 14.7 g/l hypophosphite ions); 14 g/l
malic acid; 9 g/l a oe t~c acid; and the pH of each bath was
adjusted to about 5 using ammonium hydroxide. A separate
stabilizing agent of the types heretofore employed was added to
each bath in acoordan oe with the tabulation as set forth in
Table 1.
~E 1
Rate of Depo- Percent P
Stabilizer Gonc., mq/l sition, mil/hr. in Deposit
Lead ions 0.5 0.68 9.4
Thicurea 3.0 1.1 6.2
` T ~ roprionic 3.0 1.3 9.6
Acid
~2Z5501
The lead ion oon oe ntration as set forth in Table 1 is
equivalent to 2.4 micro mols/l; the ooncentration of the thiourea
stabilizer is equivalent to 39.4 micro mDls/l; the con oe ntration
oS the thiodiproprionic acid stabilizer is equivalent to 16.8
micro mDls/l.
The temperature of each sample plating bath was adjusted
to about 88 to about 90C. Cleaned stainless steel panels (39
cm2 area) were immersed in each bath and were prelLminarily
electroplated for 30 seconds while cathodically charged to
initiate chemical deposition on the stainless steel. Thereafter,
electroless deposition of the nickel-phosphorus alloy was
oontinued for a total of 60 minutes. The resultant
nickel-phosphorus alloy deposits were separated from the stainless
steel substrates and the foils were measured for thickness and
were analyzed for phosphorus content. The rate of deposition in
terms of mils per hour and the percentage phosphorus in the nickel
alloy deposit are set forth in Table 1,
The data as set forth in Table 1 clearly demonstrates
that both thiourea and thiodiproprionic acid are effective in
increasing the rate of deposition of the nickel alloy deposit in
co~earison to the bath sample in which lead ions are the only
stabilizer. However, while both thiourea and thiodiproprionic
acid are thio compounds, only the thiourea stabilizer lowers the
percentage of phosphorus in the nickel alloy deposit. The
phosphorus content in the nickel alloy deposit when employing lead
ions or thiodiproprionic acid stabilizers is in exoe ss of 9
percent by weight.
';
16
lZ2S501
EXAMPLE 2
A series of 500 milliliter electroless nichel plating
baths was prepared containing 27 g/l nickel sulfate hexahydrate;
30 g/l sodium hypophosphite monohydrate (equivalent to 18.4 g/l
hypcphosphite ions); 26 g/l lactic acid; 9 g/l acetic acid; and
the pH was adiusted to about 4.9 employing amm~nium hydroxide. Tb
each bath sample a controlled amLunt of a thiourea stabilizing
agent or a sulfonium ~etaine compound oo~prising 3-S-isothiuronium
propane sulfonate was added in accordance with the concentrations
as set forth in Table 2. One sample bath was devoid of any
stabilizing agent to serve as a control.
~AELE 2
Con oe ntration, Rate of Depo- Percent P
Stabilizermicromols/l sition, mil/hr. in Deposit
None - 0.80 ~ 9.90
m iourea 13.1 1.00 6.67
m iourea 26.3 1.25 6.76
Thiourea 39.4 1.10 6.83
Thiourea 52.6 1.02 6.49
m iourea 65.7 1.03 6.46
Thiourea 78.9 zero
3-S-iso- 6.3 1.10 8.22
thiuronium
propane 31.6 1.10 7.36
sulfonate
" 63.2 1.23 6.36
94 9 ~.00 6.29
" 110.7 1.12 5.95
" 126.5 1.02 5.42
n 142.4 1.05 5.34
" 158.2 zero
Cleaned stainless steel test panels were plated in
accordan oe with the procedure descri~ed in Example 1 employing a
bath operating temperature of about 88 to about 90C for a period
of 60 minutes following an initial 30 second electrolytic
deposition on each test panel to initiate deposition. The
~ lZZ55(~1
resulting nickel-phosphorus alloy deposit produced from each bath
sample was remcved as a foil from the test panel and the foils
were measured with a dial micrometer to determine the deposition
rate and were also analyzed for the percentage of phosphorus in
the deposit. The results are also set forth in Table 2.
As will be apparent from the data set forth in Table 2,
both the thiourea stabilizing agent and the sulfonium betaine
compound additiv~ provide an increase in the rate of deposition of
the nickel-phosphorus deposit in oomparison to the same bath
devoid of any stabilizing additive agent. Hbwever, it will be
noted that the useful operating range of the sulfonium betaine
compourd is m~re than twice that of the thiourea stabilizing agent
providing for substantial simplicity in the maintenance and
control of the plating bath during commercial operation.
Furthermore, the percentage of phosphorus in the deposit obtained
from the baths employing the sulfonium betaine compound in
a~rdance with the practi oe of the present invention attains a
value of m~re than 17 percent less than that obtained employing
the thiourea stabilizing agent.
EXAMPLE 3
A six liter electroless nickel plating bath was prepared
oontaining 27 g/l nickel sulfate hexahydrate; 30 g/l sodium
hypophosphite monohydrate; 35 g/l lactic acid; 1.5 g/l succinic
acid; 0.5 g/l tartaric acid; 1 mg/l lead ions and 1 mg/l cadmium
ions in further oombination with 60 to 130 micro mols/1 of a
sulfonium betaine oompound oomprising 3-S-isothiuronium propane
18
J.22S5(~1
sulfonate. The bath was adjusted and maintained at a pH of about
4.2 to about 5.2 employing a~mDnium hydroxide and at a te~perature
ranging from abcut 85 to about 95C for a prolonged test. The
bath was periodically replenished to maintain the nickel and
hypophosphite ion oon oe ntration substantially constant for more
tihm 8 bath turnovers. A bath "turnc~er" or bath cycle is defined
as a plating duration when all of the original nickel metal
content in the bath has been consu~ed and has been replenished by
subsequent additions. Generally, the useful operating life of
electroless nickel plating baths in accordance with prior art
practice ranges from about 6 to about 10 turnovers before the bath
must be discarded.
At various times during the operating life of the bath,
test panels were plated in the bath in accordance with the
procedure as set forth in Example 1 and the nickel-phosphorus
alloy deposits were analyzed for percentage of phosphorus as well
as the deposition rate of the nickel alloy deposit. me results
obtained are set forth in Table 3.
TABLE 3
Bath Eath Bath Temp. Deposition Per oe nt P
TurnoverspH C Rate mils/hr. in ~eposit
6.4 4.6 95C O.S0 3.5
7.8 4.8 95C 0.48 2.8
8.1 4.2 85C 0.22 3.2
The data as set forth in Table 3 clearly demDnstrate the
very low per oe ntages of phosphorus in the nickel alloy deposit
19
lZ2~SOi
which are obtainable employing an electroless nickel plating bath
prepared in acoordan oe with the present invention employ m g
concentrations and operating conditions typical of those employed
commercially. It is anticipated from prior testing that if only
lead ions and cadmium ions had been employed as stabilizer agents
without the presen oe of the sulfonium betaine ~ uund, the nickel
alloy deposit would have contained in excess of about 9 to 10
percent phosphorus particularly when operating at a pH of ~bout
4.2. In oontrast, the use of the sulfonium betaine compound
provided nickel alloy deposits which were well und~r 4 percent by
weight phcsphorus. It will be further noted, that the bath of
Example 3 is simple to control because of the relatively broad
effective operating range of con oe ntration permissible by the
3-S-isothiuronium propane sulfonate additive compound.
EXAMPLE 4
A 500 milliliter electroless nickel plating bath was
,orepared using 27 g/l of nickel sulfate hexahydrate (equivalent to
6 g/l of nickel ions); 30 g/l of sodium hypophosphite monohydrate
(equivalent to 18.4 g/l of hypophosphite ions); 31 g/l of lactic
acid; 2 g/l of malic acid; 0.6 g/l of citric acid; 0.00237 g/l of
cadmium a oe tate dihydrate (equivalent to 1 mg/l of cadmium ions)
and sufficient ammonium hydroxide to produce a bath pH of about
5Ø Tb the ~ath was then added 0.176 g/l of antimony potassium
tartrate trihydrate; -Sb2K2C8H4012-3H20- (equivalent to 64 mg/l of
antimony ions). ~en this plating bath was heated to 90C, and a
cleaned steel panel (80 cm2 surfa oe area) was immersed in the
`` lZ2550~;
bath, it was found that a deposit of nickel could not be obtained
because the bath was over stabilized with antimony and cadmium
ions.
To the foregoing bath, 8 mg/l (40.4 micro mol/l) of
3-S-isothiuronium propane sulfonate was added. When this bath was
heated to 90C and a steel panel ~ 80 cm2 surfa oe area) was
immersed in it, an excellent nickel deposit was obtained. The
nickel deposition rate was about 1.3 mil/hr. for a thirty minute
deposit. The deposit was analyzed and contained 7.95 percent by
weight phosphorus.
This example demonstrates that the sulfonium betaine
ocmpounds of this invention can also rejuvenate an otherwise over
stabilized electroless nickel plating bath to restore it to
satisfactory operation condition.
EXAMPLE 5
Four 500 ml electroless nickel plating baths were
prepared using 27 g/l nickel sulfate hexahydrate, 30 g/l sodium
hypophosphite, 31 g/l lactic acid, 2 g/l malic acid and sufficient
ammcnium hydroxide to provide a bath pH of about 5Ø Each bath
additionally oontained 2 mg/l of lead ions and 3 mg/l of cadmium
ions to stabilize the bath and brighten the nickel alloy deposits.
m e lead and cadmium ions were added as the acetate salts. TD
these four baths, various oonoentrations of 3-S-isothiuronium
propane sulfonate were added and were thereafter heated to between
85 and 90C. Stainless steel panels (80 cm2 surfa oe area),
previously given a 15 second Watts nickel strike to insure plating
21
225;5~1
on the stainless steel and easy subsequent remc~al of the deposits
for analysis, were then plated for 30 minutes. The plating
results are summarized in Table 4.
T~LE 4
Concentration of
3-S-isothiuronium
propane sulfonate neposition Percent by wt. P
(micro mol/liter Rate ~mil/hr.) in ~eposit
Zero No deposit No deposit
20.2 1.0 7.0
40.4 1.0 5.2
60.6 0.9 3.8
The bath that did not contain the sulfonium betaine
compound did not produce an electroless nickel deposit because the
bath was over stabilized with lead and cadmium. Additions of the
sulfonium betaine overcame the excessive concentration of metallic
stabilizers, and satisfactory deposits were obtained with good
rates of deposition. This example further demonstrates that
increasing the concentration of the sulfonium betai~e may decrease
the percentage of phosphorus in the deposit so that the desired
amDunt of phosphorus can be obtained by controlling the
concentration of sulfonium betaine in the plating bath.
EXAMPLE 6
Five 500 ml electroless nickel plating baths were
prepared using the bath formulation as listed in Example 5. In
place of the lead, however, 16 mg/l of antimony (added as antimony
~50I
.
potassium tartrate) were employed as a metallic stabilizer while
the cadmium ion ~added as the acetate salt) oonoe ntration was 1
mg/l. Four of the baths additionally oontained various
conoentrations of 3-S-isothiuronium prcQane sulfonate.
Nickel-phosphorus alloy deposits were obtained using the procedure
outlined in Example 5 so that deposition rate and phosphorus
oontent oould be medsured. Table 5 summarizes the results of
these tests.
TABLE 5
Concentration of
3-S-isothiuronium
proFane sulfonate Deposition Percent by wt. P
(micro mDl/liter) Rate (mil/hr.) in Deposit
Zero l.0 10.6
20.2 1.0 7.16
40.4 l.0 6.69
60.6 ~.0 6.82
80.8 No deposit No deposit
The above data demDnstrates that when 3-S-isothiuronium
propane sulfanate is used in combination with antimDny rather than
lead as the metallic stabilizer, the phosphorus oontent of the
resulting electroless nickel deposits does not continue to
decrease with increasing ooncentrations of the sulfonium betaine,
but rather remains fairly constant at about 7 percent. miS
feature is advantageous in commercial practice as wide variations
in sulfonium betaine ooncentration do not result in appreciable
changes in the phosphorus aontent of the nickel-alloy deposit.
-
EX oe ssive amounts of the sulfonium betaine compound can over
stabilize the bath and should normally be avoided. me actual
concentration that prevents nickel deposition varies, depending cn
the basic bath composition as well as the ooncentration and kinds
of other metal ions present in the bath.
E~U~LE 7
An aqueous acidic electroless nickel plating bath is
prepared containing the following oonstituents:
Constituent g/l
NiS04 6H2 27
NaH2P02 2
Malic acid 15
Lactic acid 10
Citric acid o.s
Sblll 0.010
Pb++ 0.0005
Cd++ 0.001
3-S-isothiuronium 0.005
propane sulfonate
NH40H - to give pH 4.6 - 5.2
The bath is operated at a temperature of about 75 to
about 95C.
The concentrations of the various bath components may be
varied up or down by at least 25 percent without seriously
impairing the efficacy of the system. Likewise, simple
substitutions can be made. For example, scdium hydroxide may be
24
.
lZZ~
used in place of amm~nium hydroxide if an ammonia free bath is
desired b~cause of environmental considerations. Potassium
hypophosphite may be used in place of sodium hypophosphite.
Sodium, pokassium, ammDnium and si~llar salts of the oomplexor
acids may be used rather than the paren~ acids. Likewise, the
metallic stabilizers may be added as the salt of any bath
compatible anion, such as acetate, tartrate, propionate, etc.
EXAMPLE 8
A 500 ml alkaline electroless nickel bath was prepared
containing 26 g/l nickel chloride hexahydrate (equivalent to 6.4
g/1 of nickel ions), 15 g/l of sodium hypophosphite (equivalent to
~.2 g/l of hypophosphite ions), 50 g/l of ammcnium chloride, 60
g/l of diammonium hydrogen citrate, and 0.003 g/l of
3-S-isothiuronium propane sulfonate ~equivalent to 15 micro
mol/l). The pH was adjusted to 8.5 and the bath was operated at a
temperature of 80 to 85C.
A test panel oomprising a nonconductive polymeric
material was subjected to a conventional pretreatment to render
the polymeric substrate receptive to a ~subsequent electroless
nickel plating. Suc-h pretreatment as well known in the art
includ~es cleaning, etching, neutralization, and subseouent
activation employing an aqueous acidic solution containing a
tir.-palladium oomplex to foDm active sites on the substrate
generally followed by an accelerating treatment. The resultant
pretreated polymeric test panel was immersed in the alkaline bath
for a pnriod of 30 minutes. An inspection of the plated test
0~
panel revealed a r.ickel alloy deposit containing about 3 percent
by weight phosphorus. The rate of deposition was about 0.2 mil
per hour. While t~is deposition rate is oomparatively low
cor~xLn~d to mDst acidic electroless nickel plating kaths, it is
generally adequate for many applications such as plating on
plastics, glass and other nonmetallic substrates.
It was also observed that further additions of the
sulfonium betaine compound to the alkaline bath caused a cessation
in the deposition of the nickel alloy deposit when the
concentration attained about 25 micro mol per liter. The maximum
permissible concentration of the sulfonium betaine oompound in
such aLkaline electroless nickel baths will vary depending upon
the specific chemistry of the bath and the types and
concentrations of other constituents present. Generally, the
l~Pful cperating ooncentration range of the sulfonium betaine
co~pound is lower for alkaline electroless nickel plating baths
than for acidic electroless nickel plating baths.
EXAMoeLE 9
A series of electroless nickel plating baths is prepared
containing nickel ions in a concentration ranging from about 1 up
to about 15 gtl; hypophosphite ions in a concentration ranging
fram about 2 up to about 40 g/l; ccmplexing agents present in
amounts up to about 200 g/l including glycolic acid, lactic acid,
malic acid, glycine, citric acid, acetic acid, tartaric acid,
succinic acid as well as the bath soluble and oompatible salts and
mixtures thereof; a bath soluble and ccmpatible wetting agent in
26
~Z255~1
amounts up to about 1 g/l; stabilizing metal ions including lead,
cadmium, tin, bis~uth, antimcny, zinc and mixtures thereof in
conoentrations up to about 100 ppm; other stabilizing agents
including cyanide, thiocyanate and simiLar well known stabilizing
ions in anounts up to abcut 2D ppm; and one or a mQxture of
sulfonium betaine compounds including 3-S-isothiuranium propane
sulfonate, N,N'-dimethyl-3-S-isothiuronium propane sulfonate,
N,N'-diethyl-3-S-isothiuronium propane sulfonate,
N,N'-dihydroxymethyl-3-S-iscthiuronium propane sulfonate,
N,N'~diisapropy1-3-S-isothiuranium propane sulfonate,
N,N,N',N'-tetr~.~thyl-3-S-isothiuronium prapane sulfonate,
N,N,N'-trimethyl-3-S-isothiuronium propane sulfonate,
2-S-isothiuronium ethane sulfonate, 3-S-isothiuronium propane-2-ol
sulfonate as well as mixtures thereof in con oe ntrations ranging
from about 1 up to about 200 micro mol per liter and hydrogen or
hydroxyl ions to provide a bath pH ranging from about 4 to about 7
on the acid side and fram about 7 to about 10 on the alkaline
side. The specific constituents were varied within the foregoing
ranges to provide for cptimum deposition of the nickel-phosphorus
alloy in accordan oe with the intended deposit desired.
Test panels were plated in the series of baths
maintained at temperatures ranging fram about 40 up to boiling
for time periods as short as about 1 mQnute to times of several
days to achieve the desired deposit thickness. Many of the baths
were operated employing mild agitation.
Satisfactory nickel alloy deposits were obtained.
~2Z~
While it will be apparent that the preferred embodi~ents
of the invention disclosed are well calculated to fulfill the
objects above stat~d, it will be appreciated that the invention is
susceptible to modification, variation and change withDut
departing from the proper soope or fair meaning of the subjoined
claims.
28
